1. Field of the Invention
This invention relates to a novel synthetic oil for use as a thermal medium oil having excellent oxidation stability or for use as the main component for a synthetic lubricating oil having excellent oxidation stability. More particularly, it relates to such a novel synthetic oil which consists of, or comprises as the main component, a mixture of monoalkylnaphthalenes having a specific structure.
2. Prior art
With the recent remarkable progress in the chemical industry, an indirect heating system using an oil or the like therein as the thermal medium has been widely used, instead of a direct heating system, in all the fields of fiber, paper, foodstuff, architecture, chemical and like industries.
A thermal medium oil has most generally been used as the thermal medium in the indirect heating system and is required to have the following properties:
(1) excellent thermal stability PA1 (2) low vapor pressure and high flash point PA1 (3) good fluidity at low temperatures PA1 (4) nonpoisonousness and adorlessness PA1 (5) high heating efficiency
As such thermal medium oils, there are now widely used, for example, not only antioxidantincorporated highly refined mineral oils but also phenyl ethers, polyphenyls, arylalkanes and alkylnaphthalenes having a methyl, ethyl, propyl or like group.
Among the above thermal medium oils, those of the alkylnaphthalene type preferably have favorable properties such as nonpoisonousness, a low viscosity, low melting point and high boiling point. However, they are still not satisfactory in stability to oxidation.
Lubricating oils are generally required to have a long term service life. To meet this requirement, there has usually been used a lubricating oil prepared by adding, as required , a suitable antioxidant to a highly refined mineral oil. It is difficult, however, to use a mineral oil as a lubricant for a long period of time under severe temperature conditions since the mineral oil has limited oxidation stability. Thus, as lubricating oils having better oxidation stability, there have been developed and widely used ester-type synthetic oils such as diesters and polyol esters, and hydrocarbon-type synthetic oils such as poly-.alpha.-olefins and alkylbenzenes.
However, although these known synthetic lubricating oils are appreciated to have higher oxidation stability than mineral oils, they are still not satisfactory in stability to oxidation.
The present inventors made intensive studies in attempts to develop synthetic oils having further higher oxidation stability which are satisfactory for use as a thermal medium oil or the main component of a synthetic lubricating oil and, as the result of their studies found that synthetic oils consisting of, or comprising as the main component, a mixture of monoalkylnaphthalenes having a specific structure, show remarkably high oxidation stability as compared with the conventional known systhetic oils. The synthetic oils so found may be used as a satisfactory synthetic lubricating oil or thermal medium oil.
This invention is based on this finding or discovery.